Flowthrough water heaters are used to heat water in a wide variety of applications, such as spas, hot tubs and pools. A typical water heater includes an electric heating element contained within a fabricated stainless steel heater housing. A section of the housing may be flattened to define a mounting surface through which the heating element is installed and electrically connected. At least one aperture extends through the mounting surface, and is sized to receive the terminal ends of the heating element therethrough. The heating element is constructed from a heating coil that has two terminal ends, each connected to a cold pin. The heating coil and the cold pins are coaxially housed within a tubular outer sheath of stainless steel and the sheath is filled with a dielectric material. An annular metal bulkhead flange is typically braised or welded about the outer sheath, adjacent each end of the element.
Mounting of the heating element is typically completed by placing a gasket on the bulkhead and then securing a nut to the exterior of the terminal ends to compress the gasket. As mounted to the housing, the heating element is submerged directly into the water. As water passes through the housing, heat is transferred to the water from the heating elements. thereby causing the water to increase in temperature. Although submerging heating elements within water is an effective method of heating water, it is not without its problems.
First, because the heating elements are submerged within the water, they are subject to failure due to corrosion. Additionally, because such water heaters are used in an environment where the water is chemically treated, corrosion mechanisms include galvanic corrosion, chemical pitting and electrochemical corrosion. Further, because the welded or brazed joint between the bulkhead flange and outer sheath of the heating element is typically submerged in water, it too is subject to failure due to corrosion. When the element or a weld fails, the entire heater assembly typically must be replaced. Finally, as a result of holes being drilled through the housing for attachment of the heating elements, such water heaters are also expensive to manufacture. Any changes in the number or size of the heating elements in a conventional water heater, or the flow capacity of the heater housing, requires a specially sized and/or drilled housing.
Thus, there exists a need for a water heater that not only has a high degree of corrosion resistance, but is also economical to manufacture and maintain.